Antigen-nonspecific intercellular interactions between T-lymphocytes and antigen-presenting cells (APCs) generate T cell costimulatory signals that generate T cell responses to antigen (Jenkins and Johnson (1993) Curr. Opin. Immunol. 5:361-367). Costimulatory signals determine the magnitude of a T cell response to antigen, and whether this response activates or inactivates subsequent responses to antigen (Mueller et al. (1989) Annu. Rev. Immunol. 7: 445-480).
T cell activation in the absence of costimulation results in an aborted or anergic T cell response (Schwartz, R. H. (1992) Cell 71:1065-1068). One key costimulatory signal is provided by interaction of the T cell surface receptor CD28 with B7 related molecules on antigen presenting cells (e.g., also known as B7-1 and B7-2, or CD80 and CD86, respectively) (P. Linsley and J. Ledbetter (1993) Annu. Rev. Immunol. 11: 191-212).
The molecule now known as CD80 (B7-1) was originally described as a human B cell-associated activation antigen (Yokochi, T. et al. (1981) J. Immunol. 128:823-827; Freeman, G. J. et al. (1989) J. Immunol. 143:2714-2722), and subsequently identified as a counterreceptor for the related T cell molecules CD28 and CTLA4 (Linsley, P., et al. (1990) Proc. Natl. Acad. Sci. USA 87:5031-5035; Linsley, P. S. et al. (1991a) J. Exp. Med. 173:721-730; Linsley, P. S. et al. (1991b) J. Exp. Med. 174:561-570).
More recently, another counterreceptor for CTLA4 was identified on antigen presenting cells (Azuma, N. et al. (1993) Nature 366:76-79; Freeman (1993a) Science 262:909-911; Freeman, G. J. et al. (1993b) J. Exp. Med. 178:2185-2192; Hathcock, K. L. S., et al. (1994) J. Exp. Med. 180:631-640; Lenschow, D. J. et al., (1993) Proc. Natl. Acad. Sci. USA 90:11054-11058; Ravi-Wolf, Z., et al. (1993) Proc. Natl. Acad. Sci. USA 90:11182-11186; Wu, Y. et al. (1993) J. Exp. Med. 178:1789-1793). This molecule, now known as CD86 (Caux, C., et al. (1994) J. Exp. Med. 180:1841-1848), but also called B7-0 (Azuma et al., (1993), supra) or B7-2 (Freeman et al., (1993a), supra), shares about 25% sequence identity with CD80 in its extracellular region (Azuma et al., (1993), supra; Freeman et al., (1993a), supra, (1993b), supra). CD86-transfected cells trigger CD28-mediated T cell responses (Azuma et al., (1993), supra; Freeman et al., (1993a), (1993b), supra).
Comparisons of expression of CD80 and CD86 have been the subject of several studies (Azuma et al. (1993), supra; Hathcock et al., (1994) supra; Larsen, C. P., et al. (1994) J. Immunol. 152:5208-5219; Stack, R. M., et al., (1994) J. Immunol. 152:5723-5733). Current data indicate that expression of CD80 and CD86 are regulated differently, and suggest that CD86 expression tends to precede CD80 expression during an immune response.
Soluble forms of CD28 and CTLA4 have been constructed by fusing variable (v)-like extracellular domains of CD28 and CTLA4 to immunoglobulin (Ig) constant domains resulting in CD28Ig and CTLA4Ig. CTLA4Ig binds both CD80 positive and CD86 positive cells more strongly than CD28Ig (Linsley, P. et al. (1994) Immunity 1:793-80). Many T cell-dependent immune responses are blocked by CTLA4Ig both in vitro and in vivo. (Linsley, et al., (1991b), supra; Linsley, P. S. et al., (1992a) Science 257:792-795; Linsley, P. S. et al., (1992b) J. Exp. Med. 176:1595-1604; Lenschow, D. J. et al. (1992), Science 257:789-792; Tan, P. et al., (1992) J. Exp. Med. 177:165-173; Turka, L. A., (1992) Proc. Natl. Acad. Sci. USA 89:11102-11105).
Peach et al., (J. Exp. Med. (1994) 180:2049-2058) identified regions in the CTLA4 extracellular domain which are important for strong binding to CD80. Specifically, a hexapeptide motif (MYPPPY, SEQ ID NO: 9) in the complementarity determining region 3 (CDR3)-like region was identified as fully conserved in all CD28 and CTLA4 family members. Alanine scanning mutagenesis through the MYPPPY motif (SEQ ID NO: 9) in CTLA4 and at selected residues in CD28Ig reduced or abolished binding to CD80.
Chimeric molecules interchanging homologous regions of CTLA4 and CD28 were also constructed. Molecules HS4, HS4-A and HS4-B were constructed by grafting CDR3-like regions of CTLA4, which also included a portion carboxy terminally, extended to include certain nonconserved amino acid residues onto CD28Ig. These homologue mutants showed higher binding avidity to CD80 than did CD28Ig.
In another group of chimeric homologue mutants, the CDR1-like region of CTLA4, which is not conserved in CD28 and is predicted to be spatially adjacent to the CDR3-like region, was grafted, into HS4 and HS4-A. These chimeric homologue mutant molecules (designated HS7 and HS8) demonstrated even greater binding avidity for CD80 than did CD28Ig.
Chimeric homologue mutant molecules were also made by grafting into HS7 and HS8 the CDR2-like region of CTLA4, but this combination did not further improve the binding avidity for CD80. Thus, the MYPPPY motif of CTLA4 and CD28 was determined to be critical for binding to CD80, but certain non-conserved amino acid residues in the CDR1- and CDR3-like regions of CTLA4 were also responsible for increased binding avidity of CTLA4 with CD80.
CTLA4Ig was shown to effectively block CD80-associated T cell co-stimulation but was not as effective at blocking CD86-associated responses. Soluble CTLA4 mutant molecules, especially those having a higher avidity for CD86 than wild type CTLA4, were constructed as possibly better able to block the priming of antigen specific activated cells than CTLA4Ig.
There remains a need for improved CTLA4 molecules to provide better pharmaceutical compositions for immune suppression and cancer treatment than previously known soluble forms of CTLA4.